Ducted fan gas turbine engines suitable for aircraft propulsion are frequently provided with a thrust reverser. Typically the thrust reverser is a mechanical device which, when operative, deflects at least a portion of the exhaust efflux of the engine in such a direction as to provide retardation of the aircraft carrying the engine. Such retardation is necessary in order to economise on the use of frictional braking systems when the aircraft carrying the engine lands. Frictional braking systems, which are normally in the form of disc brakes fitted to the aircraft's wheels, are subject to expensive wear as are the tires of the braked wheels.
While thrust reversers are an attractive alternative to frictional methods of aircraft braking, they are potentially hazardous to flight safety if they are inadvertently deployed during flight. Measures are therefore taken to ensure that the possibility of this happening is minimised. Typically, such measures include providing the aircraft's undercarriage with electrical switches which indicate whether the aircraft is on the ground. The switches are linked to the control systems of the aircrafts engines so that the thrust reversers are disabled when the aircraft is not on the ground. However, such switches and their associated equipment are not completely failsafe.